Brush holder



Aug. 4, 1 953 E. c. BALLMAN I 2,643,023

- BRUSH HOLDER Filed June 2, 1950 FIG. 4.

INVENTORZ EDWIN'C. BALLMAN ATTORNEYS.

Patented Aug. 4, 1953 BRUSH HOLDER Edwin C. Bellman, University City, Mo., assignor to Ballman Engineering Company, St. Louis, Mo., a corporation of Missouri Application June 2, 1950, Serial No. 165,703

1 Claim. 1

This invention relates to brush holders for electrical equipment of the character of dynamoelectric machines for which commutator brushes are used. Commutator brush holders known heretofore have had certain disadvantages. Brush holders of the box type have often allowed chatter because of the looseness with which the brush is held. In brush holders of the hinge type, the point of commutation changes with wear of the brush. In the reaction type brush holder, the brush tends to wedge when rotation of the commutator is reversed.

One of the objects of this invention is to provide a brush holder which is equally effective for either direction of rotation of the commutator, and in which the brush will not chatter, tip, or wedge, even when the direction of rotation of the commutator is reversed.

Another object is to provide a brush holder in which the point of commutation of the brush held remains constant for any one direction of rotation of the commutator. Other objects will be apparent to those skilled in the art when the following description is read, in connection with the accompanying drawings.

In accordance with this invention, a simple, sturdy brush holder is provided, which is equally effective for either direction of rotation of the commutator; in which the brush held will not chatter, tip or wedge, in use; in which the brush held is maintained at a constant point of commutation in either direction of rotation of the commutator; and which allows the brush to right itself if the brush is accidentally tipped, as for example when it is installed. This is accomplished by providing guides, the dimensions of which bear a peculiar relation to the width of the brush, acting in conjunction with a loaded roller which engages the free end of the brush.

Referring now to the drawings:

Figure l is a plan view showing an illustrative embodiment of this invention;

Figure 2 is a view in side elevation, partly cut away, of the illustrative embodiment shown in Figure 1;

Figure 3 is a view in side elevation, partly cut away, showing this invention as applied to another embodiment of brush; and

Figure 4 is a diagrammatic view illustrating certain dimensions involved in this invention.

Referring now to the illustrative embodiments shown in the drawings, and particularly to Figures l and 2, I represents a rectangular brush, having a curved surface 2 at its free end. Guide members 3 have guide surfaces 4. Guide surfaces 4 have upper edges 5, and lower edges 6. A roller I, rotatable about a shaft 8, is loaded with a spring 9. The shaft 8 and the roller 1 are supported by an arm I 0, to which the spring 9 is attached at a cross-bar II. The arm I0 is pivoted at I2 to an upright I3. Upright I3 is provided with a base I4, to which an end of spring 9 is secured at I5. The upright I3 is connected to guide members 3 by means of an extension I6 and a screw IT, to form a complete unit. The lower end I8 of brush I engages face I9 of commutator 20, which rotates about axis 2 I. A plate 22 is secured to the ends of guide 3 by screws 25 to prevent lateral displacement of the brush I. In the embodiment of brush shown in Figure 3, the free end of the brush I has a fiat surface 23. In the device of this invention, chattering is minimized by the pressure of loaded roller 1 on the free end of the brush I. The brush is maintained at a constant point of commutation for any one direction of rotation of the commutator and, within narrow limits, regardless of the direction of rotation of the commutator, by guides 3. The clearance between the guide surfaces 4 and the brush I is made as small as possible, consonant with free sliding of the brush between them. The brush is prevented from tipping while in use by so proportioning the height of the upper edge 5 of the guide surfaces 4 from the commutator face I9, with respect to the width of the brush, as to make tipping of the brush impossible for a given coefiicient of friction between the brush and commutator face. It has been found that this condition is met if a radially extending brush is supported against rotation at a point at a distance above the surface of the commutator represented by the formula t W/2f (1) where t is the distance from the commutator face to the highest point at which the brush is retained against rotation, W is the width of the brush, and f is the coefiicient of friction of the brush against the commutator surface. If, for example, j=.25, t 2W. Thus, in this illustration, if the point of support is at a height above the commutator surface less than twice the width of the brush, no tipping will occur. It can be seen that if this formula holds true, the limiting construction of this phase of this invention would be knife edges making contact with the brush at a height no reater than t. In fact, so far as tipping is concerned, it is possible to make use of a pair of pins P at or below the point t, as shown diagrammatically in Figure 4. However,

due to the fact that the surface of lower end [8 of the brush 1 may not always properly conform to the commutator surface, either because the brush is accidentally tipped in installing, is not yet properly worn in after installation, or because it has worn unevenly, the brush may not extend radially of the commutator, in which event the brush tends to wedge between the pins. However, by using guides 3, with guide surfaces 4, the lower edges 6 of which are close to the commutator surface, wedging is precluded. At the same time, the upper edges of guide surfaces 4 are positioned at a distance no greater than i from the commutator surface. Thus, if the brush is slightly disaligned with the guide surfaces 4 so that one side of the brush engages the guide surface 4 only at a top edge 5, tipping is still impossible, since the top edge '5 is at a distance not greater than 15 from the commutator surface.

In practice, as has been suggested, there is some appreciable clearance between the brush and the guide surfaces, to allow for free radial movement of the brush. Also, the brush is capable of being tipped manually. In order to provide for the slight shift from one guide surface to the other when the direction of rotation of the commutator is reversed, and to allow the brush to right itself if it is accidentally tipped, as for example in installing the brush, the friction of the loaded roller 1 on the free end of the brush 1 must be limited.

It has been determined that, to provide for free shifting of the brush from one guide surface to the other when the direction of rotation of the commutator is reversed, the condition represented by the following formula must be met.

rf n- Where f2 is the coefficient of friction between the loaded roller and the free end of the brush, 1 is the coefiicient of friction between the lower end of the brush and the commutator, h is the height of the upper surface of the brush from the commutator, and r is the radius of the commutator from the axis 2| to face I9. It can be seen that the value of f2 will depend in part upon how freely the roller 1 revolves about shaft 8. It has been found desirable, when the coefficient of friction f2 is high, to provide an upper surface 2, with a radius of curvature r-i-h as shown in the embodiment in Figure 2. It is to such a surface that the Formula 2 is strictly applicable. On the other hand, when the coefficient of friction i2 is made very low, as for example by making the radius of the roller 1 large as compared with that of shaft 8, so that the value of f2 approaches that of the coefficient of rolling friction for the materials used, a flat upper surface 23, as shown in Figure 3, may be used in practice.

It has been determined that to allow the brush to be self-righting, the conditions represented by the following formula must obtain:

in which f2 is again the coefiicient of friction between the loaded roller and the free end of the brush, W is the width of the brush, and h the height of the upper surface of the brush from the commutator.

Thus, in order for the brush to be both shift- 4 able and self-righting, a value of f2 must be maintained which is the lesser of the values derived from Formulae 2 and 3.

As an illustration, using dimensions which are commercially feasible, if f has a value of .25, r of 1% inches, w of inch, and h of 1 inches, then t must be less than inch, and f2 must be less than .139. Since the coefficient of friction of a steel roller with brushes of readily available commercial types is approximately .05, the problem of obtaining a suitably low value of I2 is not diflicult.

In operation, when the conditions of Equations 1, 2 and 3 are met, the brush will shift from one guide surface to another when the direction of rotation of the commutator is changed, and will right itself if it is accidentally tipped. Furthermore, the brush will bear upon the commutator over the whole face of the brush, even if the brush does not bear exactly along the guide surface, since the brush must bear against the guide somewhere at a height less than the critical. height t at which the brush can tip.

In practice, since the value of f is generally in the range .15 to .25, and the value of f2 can readily be made at least as low as .07, a general purpose brush holder of the character described can be constructed in which the distance of the upper edge of the guide surface from the commutator face is approximately twice the width of the brush.

It can be seen that the operating characteristics of this brush holder are independent of the direction of rotation of the commutator, since the roller and the guide surfaces are symmetrical with respect to the brush.

Numerous variations in detail of construction within the scope of the specification and claim will be apparent to those skilled in the art.

Thus it can be seen that a simple, sturdy brush holder is provided, which is reversible, in which chattering, tipping and wedging in use are avoided, and which allows an accidentally tipped brush to right itself.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

A holder for maintaining a brush of width 10 and radial length h, in contact with a commutator of radius r, the brush having an arcuate free end of a radius of curvature r-HL, said brush having a coefficient of friction f with the commutator surface, comprising a roller arranged to exert pressure upon the free end of said brush, said roller having a coefficient of friction f2 with respect to the free end of said brush represented by the lower of the values obtained by the formulae EDWIN C. BALLMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,614,616 Janette Jan. 18, 1927 FOREIGN PATENTS Number Country Date 438,085 Germany Dec. 8, 1926 525,907 Germany May 30, 1931 

